1. Field of the Invention
The present invention relates to the operation of switched reluctance machines from dual voltages, particularly those performing starting and generating functions for internal combustion engines.
2. Description of Related Art
It is common for vehicles with internal combustion engines to be equipped with separate electrical machines: one for starting the engine and one for generating electrical power to recharge the starting battery and to supply ancillary electrical loads on the vehicle. The starter is typically supplied from a storage battery carried on board the vehicle. A 12 volt battery is commonly used for private cars and small industrial vehicles, whereas a 6 volt system has been used for motorcycles and a 24 volt system is commonly used on larger industrial vehicles. While in principle there is no particular limit to which storage batteries could be made, it has been found economic to limit the choices to these noted above.
If the vehicle is, say, a private car, the electrical loads presented by auxiliary equipment (e.g., windscreen wipers, ventilation fans, seat adjusters, heaters, etc) is relatively small and consequently the generator required to supply these loads and to keep the battery in a state of charge so that the engine can be restarted is also relatively small, typically around 60% of the size of the starter motor. Normally the generator generates onto an electrical bus running around the vehicle to supply the electrical loads and provide charge for the battery.
Although electrical machines in general can operate in both motoring and generating modes, it has not normally been found to be cost effective to combine the starting and the generating duties to allow them to be carried out by one machine. This is because of the speeds and loads over which the two machines typically operate; the starter has to provide peak power at relatively low engine speeds, say up to 600 rev/min, whereas the generator has to operate over a wide speed range, say 700 to 6000 rev/min and be capable of providing full output over most of that range. The result is that the two machines tend to be very different in design.
However, with the trend towards greater electrical loads, especially on larger vehicles, generator sizes are increasing, so the resulting generator weight is an incentive to seek ways of combining the starting and generating functions into a single machine. One type of electrical machine which is favored for this dual role is the switched reluctance machine, since it is economical to produce yet is inherently rugged and can operate over a wide speed range. U.S. Pat. Nos. 5,489,810 and 5,493,195 to Ferreira and Heglund, respectively, both incorporated herein by reference, describe certain aspects of switched reluctance machines used as starter/generators for aircraft engines.
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. This type is generally referred to as a synchronous reluctance machine, and it may be operated as a motor or a generator. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This second type of reluctance machine is generally known as a switched reluctance machine and it may also be a motor or a generator. The characteristics of such switched reluctance machines are well known and are described in, for example, xe2x80x9cThe characteristics, design and application of switched reluctance motors and drivesxe2x80x9d by Stephenson and Blake, PCIM ""93, Nxc3xcrnberg, Jun. 21-24, 1993, incorporated herein by reference. The present invention is generally applicable to reluctance machines, particularly switched reluctance machines operating as both motors and generators.
FIG. 1 shows the principal components of a typical switched reluctance drive system. The input DC power supply 11 can be a battery or rectified and filtered AC mains for example. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the machine 12 by a power converter 13 under the control of the electronic control unit 14. Some form of current detection 17 is typically used to provide current feedback from the phase windings to the controller. The switching must be correctly synchronized to the rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms, for example it may take the form of hardware, as shown schematically in FIG. 1, or of a software algorithm which calculates the position from other monitored parameters of the drive system, as described in EP-A-0573198 (Ray), incorporated herein by reference. In some systems, the rotor position detector 15 can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter 13 is required.
The energization of the phase windings in a switched reluctance machine depends heavily on accurate detection of the angular position of the rotor. The importance of accurate signals from the rotor position detector 15 may be explained by reference to FIGS. 2 and 3, which illustrate the switching of a reluctance machine operating as a motor.
FIG. 2 generally shows a rotor pole 20 approaching a stator pole 21 according to arrow 22. As illustrated in FIG. 2, a portion 23 of a complete phase winding 16 is wound around the stator pole 21. As discussed above, when the portion of the phase winding 16 around stator pole 21 is energized, a force will be exerted on the rotor, tending to pull rotor pole 20 into alignment with stator pole 21.
FIG. 3 generally shows typical switching circuitry in the power converter 13 that controls the energization of the phase winding 16, including the portion 23 around stator pole 21. When switches 31 and 32 are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of switching circuitry are known in the art: some of these are discussed in the Stephenson and Blake paper cited above.
In general, the phase winding is energized to effect the rotation of the rotor as follows. At a first angular position of the rotor (called the xe2x80x9cturn-on anglexe2x80x9d, xcex8ON), the controller 14 provides switching signals to turn on both switching devices 31 and 32. When the switching devices 31 and 32 are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air gap which acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (mmf) which is provided by a current flowing from the DC supply through the switches 31 and 32 and the phase winding 23. In some controllers, current feedback is employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switching devices 31 and/or 32 on and off. FIG. 4(a) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle xcex8ON is often chosen to be the rotor position where the centerline of an inter-polar space on the rotor is aligned with the centerline of a stator pole, but may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected intermittently if chopping is employed) until the rotor rotates such that it reaches what is referred to as the xe2x80x9cfreewheeling anglexe2x80x9d, xcex8FW. When the rotor reaches an angular position corresponding to the freewheeling angle (e.g., the position shown in FIG. 2) one of the switches, for example 31, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the diodes 33/34 (in this example 34). During the freewheeling period, the voltage drop across the phase winding is small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the xe2x80x9cturn-off anglexe2x80x9d, xcex8OFF, (e.g. when the centerline of the rotor pole is aligned with that of the stator pole). When the rotor reaches the turn-off angle, both switches 31 and 32 are turned off and the current in phase winding 23 begins to flow through diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease.
As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a xe2x80x9csingle-pulsexe2x80x9d mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. Some systems do not use an angular period of freewheeling, i.e. switches 31 and 32 are switched on and off simultaneously. FIG. 4(b) shows a typical such single-pulse current waveform where the freewheel angle is zero. It is well known that the values of turn-on, freewheel and turn-off angles can be predetermined and stored in some suitable format for retrieval by the control system as required, or can be calculated or deduced in real time.
One development in electrical systems for vehicles, particularly those where the electrical load is high, is to use a high-voltage bus to supply these loads. Known designs for such vehicles use bus voltages between 24 and 600 volts. The use of these higher voltages provides a more efficient system since, for a given power, the currents are reduced as the bus voltage is raised. However this immediately brings a problem if the starting and generating functions are to be performed by one machine, since the storage battery used for starting the vehicle is unlikely to be higher than 24 volts. A machine designed to operate at a high voltage will not produce the necessary power as a starter motor if operated in the conventional manner from a low-voltage battery. One known solution is to interpose an up-converter between the battery and the machine to convert the battery voltage to a voltage at least near to the high bus voltage of the vehicle. This allows the electrical machine to operate at the same voltage for both starting and generating duties, thus avoiding the difficulty of designing the machine to operate on two quite different voltages, albeit at the expense of providing the up-converter. Such an arrangement for a switched reluctance machine is shown in FIG. 5, where only one phase of the machine is shown for convenience. The battery 38 supplies an up-converter 36 of known type to provide a high-voltage DC bus from which the winding(s) of the machine are excited in the conventional manner. The efficiency of the up-converter is a function of the voltage ratio it has to achieve. For example, an up-converter raising the voltage from 24 to 300V and having an output rating of around 10 kW will be of the order of 80% efficient. The losses are supplied from the storage battery and may therefore reduce the amount of energy available to start the engine. In addition, the cost of the up-converter is proportional to its power rating and this cost may be very significant. The conventional solution, therefore, has disadvantages of inefficiency and cost.
It is an object of the present invention at least to alleviate some of the problems associated with the prior art by supplying a switched reluctance machine from dual voltages.
According to one aspect of the invention there is provided a method of energizing a phase winding of a switched reluctance machine during a conduction period, including: supplying energy to the winding at a first voltage, and supplying energy to the winding subsequently within the same conduction period at a second voltage.
A circuit is provided by which the machine can operate from two voltages during a conduction period, hence providing the opportunity to reduce the duty and rating of the up-converter by using a lower voltage in the conduction period when appropriate. One embodiment of the invention eliminates the need for an up-converter. Thus, by the appropriate use of a lower voltage in the conduction period the up-converter can be a lower rated unit, if not eliminated altogether. This is a cost-effective solution to running a switched reluctance machine from a lower supply voltage in association with a relatively heavy load.